Storage system

ABSTRACT

With a RAID group not configured from a plurality of storage devices, a storage area of a storage device is provided directly to a virtual volume instead of providing a logical volume inside the RAID group to the virtual volume. That is, the storage system, upon receiving a write request with respect to a virtual storage area, first, specifies a data redundancy configuration (the number of data partitions and the number of created parities) and a RAID level set to a virtual volume including this virtual storage area. The storage system selects storage devices in the numbers in accordance with the specified RAID level and redundancy configuration for this virtual storage area. The storage system selects, from among the selected storage devices, a storage area that is not allocated to any virtual storage area, and allocates this storage area to this virtual storage area. The storage system partitions the data and writes this data together with the parity to this allocated storage area.

This is a continuation of U.S. Ser. No. 12/670,498, filed Jan. 25, 2010which is a 371 National Stage of PCT/JP2010/000166, filed on Jan. 14,2010. The entire disclosures of all these applications are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to storage control to which DynamicProvisioning (also called Thin Provisioning) is applied.

BACKGROUND ART

Storage systems configured from RAID (Redundant Array of Independent (orInexpensive) Disks) are being used widely today to safely storecorporate data. In addition, thin provisioning is being introduced toenhance the utilization efficiency of storage system capacity.

In a RAID system, a RAID group is configured from a plurality of storagedevices, and a logical volume is configured on the RAID group. In astorage system that does not use thin provisioning, the logical volumeis provided to a host, and a storage area of a storage device having acapacity that is equivalent to the entire storage capacity of thelogical volume is allocated to this logical volume from the time thelogical volume is created. By contrast, in thin provisioning, in which avirtual volume is provided to the host, a storage area of the storagedevice corresponding to the initial virtual storage area is notallocated to the virtual storage area inside this virtual volume, butrather, the storage area of the storage device that is allocated to thelogical volume is allocated only to the virtual storage area when therehas been a write request with respect to the virtual storage area. Forthis reason, only a storage device of the capacity required for the timebeing is provided when the storage system is introduced, and a storagedevice can easily be added as the need arises.

As a thin provisioning technique, Patent Literature 1 proposes a methodfor rearranging data on a plurality of storage devices in logical volumeunits in a storage system that uses thin provisioning.

Further, Patent Literature 2 proposes a method for adding storagedevices in storage device units by changing the data redundancyconfiguration after adding a storage device.

CITATION LIST Patent Literature [PTL (Patent Literature) 1]

-   Japanese Patent Application Laid-open No. 2008-234158

[PTL 2]

-   Japanese Patent Application Laid-open No. 2009-230352

SUMMARY OF INVENTION Technical Problem

In the above-mentioned Patent Literature 1, a RAID group is configuredfrom a plurality of storage devices prior to these storage devices beingallocated to the virtual volume, and data management is carried out inlogical volume units inside the RAID group. For example, data redundancyis maintained by storing the data that is inside the logical volume soas to extend across a plurality of storage devices of this RAID group.For this reason, storage device addition must be carried out by adding aplurality of storage devices during a single addition to achieve a groupof storage devices in the numbers required to configure a RAID group.

In the above-mentioned Patent Literature 2 method, the premise is that aRAID group is configured from a plurality of storage devices prior tothese storage devices being allocated to the virtual volume the same asin the above-mentioned Patent Literature 1, and that a logical volume inthis RAID group is provided to the virtual volume. In so doing, the RAIDgroup must be reconfigured after adding a single storage device, givingrise to the need to reorganize the data redundancy configuration.

An object of the present invention is to provide a method for enhancingthe flexibility and lowering the cost of storage device addition in astorage system that makes use of thin provisioning.

An object of the present invention is to suppress the influence, whichis exerted by the addition of storage devices, on business or otheroperations in a storage system to which the thin provisioning isemployed.

In addition, another object of the present invention is to provide astorage system that does not require a spare disk, i.e., a specificstorage device provided in preparation for a storage device failure.

Solution to Problem

In a storage system having a thin provisioning function, with a RAIDgroup being configured from a plurality of storage devices, a storagearea of a storage device is provided directly to a virtual volumeinstead of providing a logical volume inside the RAID group to thevirtual volume. That is, the storage system, upon receiving a writerequest with respect to a virtual storage area, first of all, specifiesthe data redundancy configuration (a number of data partitions and anumber of created parities) and the RAID level set to the virtual volumecomprising this virtual storage area. The storage system selects anumber of storage devices in accordance with the specified RAID leveland redundancy configuration for this virtual storage area. The storagesystem selects, from among the selected storage devices, a storage areathat is not allocated to a virtual storage area, and allocates thisstorage area to this virtual storage area. The storage system partitionsthe data and writes this data together with the parity to this allocatedstorage area.

Advantageous Effects of Invention

According to an embodiment of the present invention, it is possible toexpand storage capacity while holding the cost of storage deviceaddition to the minimum.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the configuration of a storage system related to anembodiment of the present invention.

FIG. 2 shows an overview of dynamic capacity expansion related to thisembodiment.

FIG. 3 shows an example of a first configuration of a DP pool.

FIG. 4 shows an example of a second configuration of the DP pool.

FIG. 5 shows a computer program and information stored in a memory 123.

FIG. 6 shows the configuration of a DP common specification table 503.

FIG. 7 shows the configuration of a pool management table 505.

FIG. 8 shows the configuration of a virtual LU management table 507.

FIG. 9 shows the configuration of a PDEV management table 509.

FIG. 10 shows the configuration of a real page management table 511.

FIG. 11 shows the configuration of a mapping management table 513.

FIG. 12 shows the configuration of a real access status table 515.

FIG. 13 shows the configuration of a virtual access status table 517.

FIG. 14 is a flowchart showing the flow of processing for creating a DPpool.

FIG. 15 is a flowchart showing the flow of processing for creating avirtual LU 201.

FIG. 16 shows an overview of the flow of a write to a virtual page.

FIG. 17 is a flowchart showing the flow of processing for writing to avirtual page.

FIG. 18 is a flowchart showing the flow of processing for reading from avirtual page.

FIG. 19 shows how data stored in accordance with a plurality of RAIDtypes is mixed together in a single DP pool.

FIG. 20 shows an example of the arrangement of data units (generic namefor data elements and parity) in a case where a PDEV allocated to asingle virtual page is selected without relying on the PDEV number.

FIG. 21 is a flowchart for processing carried out when adding a PDEV.

FIG. 22 shows a flowchart of a rearrange process.

FIG. 23 shows an example of rearranging carried out in a case where aPDEV has been added.

FIG. 24 is a flowchart of processing carried out when removing a PDEV.

FIG. 25 shows an example of rearranging based on access frequency.

FIG. 26 shows an overview of a restore process in the case of a failurein a PDEV.

FIG. 27 is a flowchart of a priority-restore type restore process.

FIG. 28 is a flowchart of a priority-replace type restore process.

DESCRIPTION OF EMBODIMENT

FIG. 1 is shows the configuration of a storage system related to anembodiment of the present invention.

One or more (for example, two) host apparatuses (hereinafter, host) 101are coupled to the storage system 103. Communication between the host101 and the storage system 103, for example, is carried out using aprotocol that conforms to either a Fibre Channel (FC) or an iSCSI (SCSIis the abbreviation for Small Computer System Interface).

The storage system 103 manages a virtual LU (LU is the abbreviation fora Logical Unit). The virtual LU is a virtual LU that conforms to DynamicProvisioning (also called Thin Provisioning) technology. The virtual LUcomprises a plurality of virtual pages (virtual storage areas). One ormore addresses (for example, a LBA (Logical Block Address) belong toeach virtual page.

The host 101 sends an I/O command for carrying out I/O (Input/Output)with respect to the virtual LU. The I/O command comprises I/Odestination information. The I/O destination information includes theLUN (Logical Unit Number) of a virtual LU and the virtual page addressof the virtual LU. The host 101 is generally a computer, but anotherstorage system may be used instead of a computer.

The storage system 103 comprises a plurality of physical storage devices(PDEV) 117, and a storage control apparatus (hereinafter referred to as“DKC”) 111 that is coupled to the plurality of PDEV 117. The DKC 111 ismultiplexed (for example, duplexed). The plurality of PDEV 117 may beconfigured from the same type of PDEV, or may be a mixture of aplurality of types of PDEV. As types of PDEV, for example, there are aHDD (Hard Disk Drive) and a SSD (Solid State Drive).

The DKC 111 receives and processes an I/O command (either a writecommand or a read command) from the host 101. The DKC, upon receiving awrite command, for example, allocates a plurality of real pages(substantial storage areas) to each of one or more write-destinationvirtual pages specified in the received write command. For the sake ofconvenience, the allocation of a real page to a virtual page will becalled “dynamic capacity expansion”.

A plurality of PDEV 117 are coupled to the DKC 111 via one or morebackend coupling devices 113. The backend coupling devices 113 aremultiplexed (for example, duplexed). As a backend coupling device 113,for example, a FC-AL (Fibre Channel-Arbitrated Loop) or a SAS (SerialAttached SCSI) expander may be used. One backend coupling device 113 iscoupled to two or more PDEV 117 of a plurality of PDEV 117.

A management apparatus 105 is coupled to at the least one DKC 111. Themanagement apparatus 105, for example, is coupled to the at the leastone DKC 111 via a LAN (Local Area Network). The management apparatus105, for example, may be a computer and may carry out various types ofsettings and references with respect to a LU.

Each DKC 111 comprises a FE-IF (front-end interface) 121, a BE-IF(backend interface) 127, and a controller. The controller, for example,comprises a transfer circuit 129, a memory 123, and an MP (amicroprocessor) 125. In addition to these elements, for example, thecontroller may comprise a hardware circuit for creating parity insteadof the MP 125.

The FE-IF 121, the memory 123, the MP 125, and the BE-IF 127 are coupledto the transfer circuit 129. The transfer circuit 129 relays data and acommand exchanged among these elements 121, 123, 125, and 127.

The FE-IF 121 is an interface device that is coupled to the host 101.The FE-IF 121 comprises a plurality of communication ports, and a hostcomputer 101 is coupled to each communication port. The FE-IF receivesan I/O command from the host 101 and transfers the received I/O commandto the MP 125.

The BE-IF 127 is an interface device that is coupled to the PDEV 117.The BE-IF 127 comprises a plurality of communication ports, and a PDEV117 is coupled to each communication port via the backend couplingdevice 113.

The memory 123 is configured from one or more memories. The memory 123stores information (for example, tables) and a computer program. Thememory 123 comprises a cache area. Data that is exchanged between thehost 101 and the PDEV 117 is temporarily stored in the cache area.

The MP 125 processes an I/O command from the host 101. Further, inresponse to a request from the management apparatus 105, the MP 125 alsosets information (for example, LU-related information) in the memory 123and provides the information (for example, LU-related information)stored in the memory 123 to the management apparatus 105.

FIG. 2 shows an overview of dynamic capacity expansion related to thisembodiment.

A virtual LU 201 and a DP (Dynamic Provisioning) pool 203 are managed bythe DKC 111. The virtual LU 201 is associated with one DP pool 203, anda real page is allocated from the DP pool 203 to this virtual LU 201.

The virtual LU 201 is configured from a plurality of virtual pages, andis recognized by the host 101. A RAID level is defined for the virtualLU 201. The combination, the RAID level and capacity of the virtual LU201, for example, can be set from the management apparatus 105 (refer toFIG. 1). The RAID level here is obtained by classifying the RAIDtechnology in accordance with the function thereof, and is expressed asRAID 5 and RAID 6. For example, in RAID 5, data for which there is awrite request is partitioned into n parts (n being a natural number),one parity data is created from this data, and this partitioned data andparity are distributively written to (n+1) storage devices. Further, inRAID 6, two types of parity data are created with respect to thepartitioned n pieces of data, and this partitioned data and parity aredistributively written to (n+2) storage devices. Further, a combinationis a combination of the partition number n of the data elementsarbitrarily determined for configuring the RAID, and the number m (mbeing an integer of greater than 0) of parity determined by the RAIDlevel, and is expressed as nD+mP (or D). That is, the data written to avirtual page is partitioned into n data elements, m parity is created,and these partitioned data elements and corresponding parity aredistributively written to (n+m) storage devices.

The DP pool 203 is configured from either a portion or all of the PDEV117 of the plurality of PDEV 117 of the storage system 103. The DP pool203 is configured from a large number of real pages.

As one characteristic feature of this embodiment, the DP pool 203 maylack a RAID definition, and a real page may be provided from individualPDEV rather than from a RAID group configured from a plurality of PDEV.

Generally speaking, according to the Dynamic Provisioning (ThinProvisioning) technology, a real page allocated to a virtual LU isformed on the basis of a predefined RAID group storage space.Specifically, for example, the pool is configured from either one or aplurality of RAID groups. Each RAID group is configured from two or morePDEV on the basis of the RAID level. A real page is configured on thebasis of one RAID group, and one real page extends over two or more PDEVthat configure a RAID group. For this reason, in a write to a real pagethat is allocated to a virtual page, the write-targeted data is writtenon the basis of the RAID level and combination of the RAID group thatprovides this real page.

By contrast, according to this embodiment, there is no RAID groupdefinition, and the individual PDEV 117 that configure the DP pool 203are not RAID group-configuring members. A real page is provided from anindividual PDEV. Further, the RAID level and combination are set for thevirtual LU 201. A number of real pages determined on the basis of theRAID level and combination of this virtual LU 201 is allocated from theDP pool 203 to a virtual page of the virtual LU 201, and write-targeteddata with respect to this virtual page is written to the allocatedplurality of real pages.

According to RAID technology, two or more data elements (orwrite-targeted data and a replicate of this data) from among a pluralityof data elements that configure the write-targeted data are not storedin a single PDEV 117. For this reason, in this embodiment, two or morereal pages from a single PDEV 117 will not be allocated to the samevirtual page. That is, real pages from respectively different PDEV 117are allocated to a single virtual page. Therefore, it is preferable thata plurality of real pages be allocated to a virtual page so that theutilization rate of a plurality of PDEV belonging to a DP pool (theratio of the total capacity of the allocated real pages with respect tothe capacity of the PDEV) becomes uniform. For example, this is becausea plurality of real pages are not able to be allocated from a singlePDEV to a single virtual page even when there are lots of free realpages in this one PDEV, thereby making it likely that there will be anexcess of free real pages as a result. Further, in this case, theplurality of real pages may be allocated to a virtual page so that notthe utilization rate of the plurality of PDEV but free capacity of aplurality of PDEV becomes uniform.

There may be either one or a plurality of the virtual LU 201 and the DPpool 203. According to the example of FIG. 2, it is supposed that thereare two virtual LU #1 and #2 as the virtual LU 201, there is one DP pool#1 as the DP pool 203, and the virtual LU #1 and #2 are associated withthe DP pool #1. It is also supposed that the RAID level and combinationof the virtual LU #1 is RAID 5 (2D+1P), and that the RAID level andcombination of the virtual LU #2 is RAID 6 (4D+2P). Further, it issupposed that the DP pool #1 is configured from six PDEV #01 through#06.

RAID level and combination of virtual LU #1: According to RAID 5(2D+1P), data that is targeted for a write to virtual page V11 of thevirtual LU #1 is divided into two data elements by the DKC 111, and, inaddition, one parity is created. For this reason, three real pages R11,R22, and R31 are allocated to the virtual page V11 from the three PDEV#01 through #03. The corresponding relationship between the virtual pageV11 and the real pages R11, R22, and R31 is managed by the DKC 111.Then, two data elements are respectively written to two of the threereal pages R11, R22, and R31, and a parity is written to the remainingone real page.

RAID level and combination of virtual LU #2: According to RAID 6(4D+2P), data that is targeted for a write to virtual page V21 of thevirtual LU #2 is divided into four data elements, and, in addition, twoparities are created. For this reason, six real pages R18, R23, R33,R41, R51, and R61 are allocated to the virtual page V21 from the sixPDEV #01 through #06. The corresponding relationship between the virtualpage V21 and the real pages R18, R23, R33, R41, R51, and R61 is managedby the DKC 111. Then, four data elements are respectively written tofour of the six real pages R18, R23, R33, R41, R51, and R61, and twoparities are written to the remaining two real page.

One DP pool 203 may be configured by one or more physical storagedevices (PDEV) having the same attributes, and may also be configured byPDEV having different attributes. Attributes, for example, may includethe I/O performance or reliability (for example, the MTBF (Mean TimeBetween Failures)) of the PDEV. The I/O performance of the PDEV, forexample, may include the type of the PDEV itself (for example, HDD, SSD,and so forth), the type of interface (for example, an FC (FibreChannel), a SATA (Serial ATA), an SAS (Serial Attached SCSI), and therotational speed of the disk.

As shown in FIG. 3, a single DP pool 203 may be configured from PDEV (inthe example of FIG. 3, HDD and SSD) having different types ofattributes. In a case where a virtual page is configured from aplurality of real pages having different attributes, performance dropsto the level of the PDEV with the lowest I/O performance when writing orreading data to or from the respective PDEV, making it preferable thateach virtual page be configured from PDEV having the same attribute.However, respective virtual LU are assigned to one pool. For thisreason, the mixing together of PDEV with different attributes inside thesame pool makes it possible to allocate real pages inside PDEV withdifferent attributes to each virtual page of the same virtual LU.Further, even in a case where real pages from PDEV having the sameattribute are allocated to the same virtual page, it is possible toallocate real pages from PDEV having different attributes when thecapacity of a PDEV of a certain attribute becomes insufficient. Afteradding a PDEV with this capacity storage attribute, it is possible toallocate to the added PDEV data elements allocated to PDEV withdifferent attributes. A DP pool 203 of this type of configuration makesit possible to efficiently use disk capacity.

Further, as shown in FIG. 4, one or more prescribed types of attributesof a plurality of attributes may be configured in the same PDEV of oneDP pool 203. According to the example of FIG. 4, a single DP pool 203 isconfigured from PDEV for which the PDEV performance, the PDEV interfaceand the type of the PDEV itself are all the same. According to a DP pool203 of this type configuration, it is possible to anticipate a desiredI/O performance with respect to accessing the virtual LU 201. Forexample, in a case where it is preferable to improve the I/O performanceof a virtual LU that is associated with the DP Pool #E, this virtual LUmay be associated with a DP pool (for example, DP pool #D) configuredfrom PDEV having improved I/O performance than the PDEV that configurethe DP pool #E. Furthermore, in a case where the DP pool with which thevirtual LU is associated is changed to a different DP pool, the dataelements and parity are migrated from the pre-change DP pool to thepost-change DP pool.

Information related to the DP pool 203 and information denoting thecorresponding relationship between a virtual page and a plurality ofreal pages is managed by the DKC 111. This information, for example, isstored in the memory 123 (for example, the cache area) of the DKC 111.

FIG. 5 shows the computer program and information stored in the memory123.

The memory 123 stores a control processing program 501. The controlprocessing program 501 is executed by the MP 125. In the followingexplanation, processing that is carried out by the program 501 isactually performed by the MP 125.

The memory 123 stores a DP common specification table 503, a poolmanagement table 505, a virtual LU management table 507, a PDEVmanagement table 509, a real page management table 511, a mappingmanagement table 513, a real access status table 515, and a virtualaccess status table 517. The real page management table 511 exists ineach DP pool 203. The information in at least one of these tables may bemanaged in a structure other than a table.

FIG. 6 shows the configuration of the DP common specification table 503.

The DP common specification table 503 comprises common informationrelated to the specification of the DP pool. Specifically, for example,the table 503 comprises an item 200 and a value 201 for each itemrelated to the DP pool specification. The item 200 is the name of anitem, and the value 201 is the value for this item. According to theexample of FIG. 6, the maximum number of creatable pools (the maximumnumber of DP pools 203 capable of being created in the storage system103) is 50, and the real page capacity is 32 KB (kilobytes).

Furthermore, in this embodiment, the real page capacity is the same inall of the DP pools. For this reason, the capacity of a virtual pagethat configures the virtual LU 201 is determined on the basis of thereal page capacity, RAID level and combination. When the combination isnD+mP, it is possible to store in the virtual page data of n-times thecapacity of the real page, and (n+m) times the capacity is allocated.For example, the capacity of a virtual page of a RAID 6 (4D+2P) virtualLU 201 is twice the capacity of a virtual page of a RAID 5 (2D+1P)virtual LU 201. As explained hereinabove, this is because the real pagecapacity is the same, and the number of real pages allocated to avirtual page of a RAID 6 (4D+2P) virtual LU 201 is twice the number ofreal pages that is allocated to a virtual page of a RAID 5 (2D+1P)virtual LU 201. Specifically, six real pages are allocated to thevirtual page of the RAID 6 (4D+2P) virtual LU 201, and three real pagesare allocated to the virtual page of the RAID 5 (2D+1P) virtual LU 201.

FIG. 7 shows the configuration of the pool management table 505.

The pool management table 505 comprises information related to the DPpool 203. Specifically, for example, the table 505 comprises a poolnumber 210, a PDEV number list 211, a real allocation capacity 212, autilization capacity 213, and a first PDEV number 214 for each DP pool203.

The pool number 210 is the number of the DP pool 203.

The PDEV number list 211 is a list of numbers of the PDEV that configurethe DP pool 203.

The real allocation capacity 212 is the total capacity of the PDEV thatconfigure the DP pool 203. Therefore, in a case where a new PDEV isadded to the DP pool 203, the real allocation capacity 212 will increasein proportion to the capacity of the added PDEV. By contrast, in a casewhere a PDEV is removed from the DP pool 203, the real allocationcapacity 212 will decrease in proportion to the capacity of the removedPDEV.

The utilization capacity 213 is the total capacity of a real page thathas been allocated from the DP pool 203.

The first PDEV number 214 is the number of the PDEV that is the nextallocation source of a real page. For example, in this embodiment, thetwo or more PDEV that provide two or more real pages that are allocatedto a virtual page are in PDEV number order. Specifically, for example,in a case where the DP pool is configured from the four PDEV #01 through#04, the RAID level and combination of the virtual LU is RAID 5 (2D+1P),and, in addition, an initial write is generated to an unallocatedvirtual page (a virtual page to which a real page is not allocated) inthis virtual LU, the first PDEV number 214 is “1”, and therefore,respective real pages are allocated from PDEV #01, #02, and #03 to thewrite-destination virtual page. Thereafter, the first PDEV number 214 isupdated to “4”. Then, in a case where a second write is generated to anunallocated virtual page in this virtual LU, since the first PDEV number214 is “4”, respective real pages are allocated from PDEV #04, #01, and#02 to the write-destination virtual page. After this, the first PDEVnumber 214 is updated to “3”. In this embodiment, the two or more PDEVthat are the provision sources of the two or more real pages allocatedto the virtual page are determined based on the order of the PDEVnumbers. Furthermore, the numbers of the plurality of PDEV configuringthe DP pool may be consecutive or nonconsecutive. Further, the two ormore PDEV that are the provision sources of the two or more real pagesallocated to the virtual page may be arranged in descending order fromthe largest number rather than in ascending order from the smallestnumber. Since the two or more provision-source PDEV are determined inPDEV number order, the PDEV inside the DP pool are used equably.

FIG. 8 shows the configuration of the virtual LU management table 507.

The virtual LU management table 507 comprises information related to thevirtual LU 201, and can be used to set and check the virtual LU.Specifically, for example, the table 507 comprises a LUN 220, anassigned pool number 221, a RAID level 222, a combination 223 and acapacity 224 for each virtual LU 201.

The LUN 220 is the LUN of the virtual LU 201.

The assigned pool number 221 is the number of the DP pool (the DP poolwith which the virtual LU 201 is associated) 203 to which the virtual LU201 is assigned.

The RAID level 222 is the RAID level of the virtual LU 201.

The combination 223 is the combination of the virtual LU 201.

The capacity 224 is the capacity of the virtual LU 201.

FIG. 9 shows the configuration of the PDEV management table 509 used tocreate, for example, a DP pool.

The PDEV management table 509 comprises information related to the PDEV117. Specifically, for example, the table 509 comprises a PDEV number230, an assigned pool number 231, an IF type 232, a PDEV performance233, a capacity 234, a free capacity 235, and a device type 236 for eachPDEV 117.

The PDEV number 230 is the number of a PDEV 117.

The assigned pool number 231 is the number of the DP pool (the DP poolin which the PDEV 117 is one of the components) to which the PDEV 117 isassigned. “null” shows that the PDEV 117 is not allocated to any pool.

The IF type 232 is the type (for example, SAS, SATA, and so forth) ofthe interface of the PDEV 117.

The PDEV performance 233 is expressed in accordance with the device type236 of the PDEV 117. For example, in a case where the device type 236 ofthe PDEV 117 is HDD, the PDEV performance 233 is shown by the rotationalspeed of the disk (for example, 7,200 rpm).

The capacity 234 is the capacity of the PDEV 117, that is, the totalcapacity of the real pages that that PDEV 117 provides.

The free capacity 235 is the total capacity of the real page that is notallocated to the virtual page from among the real pages that the PDEV117 provides, but can be allocated to the virtual page.

The device type 236 is the type of the PDEV 117 itself. The device type236, for example, includes HDD and SSD.

FIG. 10 shows the configuration of the real page management table 511showing the relationship between the virtual page and the real page. Thereal page management table 511 is used when allocating a real page to avirtual page.

The real page management table 511 exists for each DP pool. The realpage management table 511 comprises information related to a real pageinside the DP pool corresponding to this table 511. Specifically, forexample, the table 511 comprises a PDEV number 240, a real page number241, a LUN 242, a virtual page number 243, and an access counter 244 foreach real page.

The PDEV number 240 is the number of the PDEV 117 that provides the realpage.

The real page number 241 is the number of the real page.

The LUN 242 is the LUN of the virtual LU 201 comprising the virtual pagethat is the allocation destination of the real page.

The virtual page number 243 is the number of the allocation-destinationvirtual page of the real page.

The access counter 244 is the number of times that an I/O (access) hasbeen carried out to the real page. Furthermore, for example, in a casewhere there is a real access status table 515 as alternative informationas in this embodiment, the access counter 244 may be eliminated.

FIG. 11 shows the configuration of the mapping management table 513used, for example, when judging whether the real page is allocated tothe virtual page that has received a write command.

The mapping management table 513 shows the corresponding relationshipbetween the virtual page and the real page. Specifically, for example,the table 513 comprises a LUN 250, a virtual page number 251, a realpage identification information 252, and an access counter 253 for eachvirtual page.

The LUN 250 is the LUN of the virtual LU 201 of the virtual page.

The virtual page number 251 is the number of the virtual page.

The real page identification information 252 is information foridentifying the real pages that are allocated to the virtual page, and,for example, is a combination of the real page numbers and the numbersof the PDEV 117 of these real pages.

The access counter 253 is the number of times that an I/O (access) hasbeen carried out with respect to the real page. Furthermore, forexample, in a case where there is a virtual access status table 517 asalternative information as in this embodiment, the access counter 253may be eliminated.

FIG. 12 shows the configuration of the real access status table 515referenced, for example, when determining a storage device for rearrangeand the like.

The real access status table 515 comprises information, which is commonto one or more DP pools 203, and which shows the access state for eachreal page. Specifically, for example, the table 515 comprises a cell foreach real page, and the cell comprises information showing the accessfrequency with respect to the real page (hereinafter, real accessfrequency information). The real access frequency information, forexample, includes information showing the usage of the real pagecorresponding to this information, and shows the write frequency and theread frequency separately. The write frequency is the number of writesper unit period, specifically, for example, the number of writes perminute, hour, day and week. The read frequency is the number of readsper unit period, specifically, for example, the number of reads perminute, hour, day and week.

In a case where one PDEV 177 is added to any DP pool 203, a recordcorresponding to this added PDEV 117 is added to the table 515. Further,in a case where one PDEV 177 is removed from any DP pool 203, the recordcorresponding to this removed PDEV 117 is removed from the table 515.

FIG. 13 shows the configuration of the virtual access status table 517referenced, for example, when determining the virtual page for rearrangeand the like.

The virtual access status table 517 comprises information denoting theaccess status of each virtual page. Specifically, for example, the table517 comprises a cell for each virtual page, and the cell comprisesinformation showing the access frequency with respect to the virtualpage (hereinafter, virtual access frequency information). The virtualaccess frequency information, for example, includes information showingthe usage of the virtual page corresponding to this information, andshows both the write frequency and the read frequency. The writefrequency is the number of writes per unit period, specifically, forexample, the number of writes per minute, hour, day and week. The readfrequency is the number of reads per unit period, specifically, forexample, the number of reads per minute, hour, day and week.

In a case where one virtual LU 201 is associated with any DP pool 203, arecord corresponding to this associated virtual LU 201 is added to thetable 517. Further, in a case where one virtual LU 201 is disassociatedfrom any DP pool 203, the record corresponding to this disassociatedvirtual LU 201 is removed from the table 517.

Furthermore, instead of showing the write frequency and the readfrequency separately, at the least one of the virtual access frequencyinformation and the above-mentioned real access frequency informationmay show the access frequency without regard for a write or a read, mayshow only the write frequency, or may show only the read frequency.

Further, at the least one of the virtual access frequency informationand the above-mentioned real access frequency information may be updatedeach time an access occurs with respect to a virtual area or a realpage, or may be updated on a predetermined schedule (for example,regularly). For example, the control processing program 501 may manage aprocessed command log in the memory 123. The command log may comprise,for example, a type, a date/time (for example, date/time received,process completion date/time, or a date/time of an I/O command timestamp), the number of the access-destination virtual page, and theidentification information of the access-destination real page (forexample, a combination of the PDEV number and the real page number) foreach processed I/O command. In a case where an I/O command has beenprocessed, the control processing program 501 may add to the command logthe type, date/time, number of the access-destination virtual page, andidentification information for the access-destination real page relatedto this I/O command. The control processing program 501 may update thevirtual access frequency information and the real access frequencyinformation based on this command log.

FIG. 14 is a flowchart showing the flow of processing for creating a DPpool.

(S10)

The control processing program 501 receives a create pool instructionfrom the management apparatus 105. The parameter of the create poolinstruction, for example, is at the least (p1) of the following (p1)through (p3). The (p1) is a parameter that must be input by the user,the (p2) and the (p3) are parameters that are input at the discretion ofthe user.

(p1) Capacity requested by the user (for example, the administrator)(user-requested pool capacity). The user-requested pool capacity differsfrom the real allocation capacity 212. This is because the realallocation capacity 212 is the total capacity of the PDEV that belong toa DP pool.(p2) Information denoting PDEV 117 attribute (PDEV information). Forexample, the device type, IF type and PDEV performance.(p3) The number assigned to a pool (pool number).

(S11)

The control processing program 501 determines whether or not the numberof DP pools 203 specified from the pool management table 505 is smallerthat the value 201 of the maximum number of creatable pools. In a casewhere the result of this determination is affirmative, S12 is performed.In a case where the result of this determination is negative, thecontrol processing program 501 sends information denoting an error tothe management apparatus 105, and the management apparatus 105 displaysan error (S16).

(S12)

The control processing program 501 selects PDEV 117 so that the totalcapacity is greater than the user-requested pool capacity. Specifically,for example, the control processing program 501 carries out thefollowing processing.

(S12-1): The control processing program 501 references the PDEVmanagement table 509, and specifies a PDEV (a PDEV that is not assignedto any DP pool 203) for which the assigned pool number 231 is “null”.The specified PDEV is a DP pool component candidate. Furthermore, in acase where the PDEV information is the parameter of the create poolinstruction, the assigned pool number 231 is “null”, and, in addition,the PDEV that conforms to this PDEV information is specified as the DPpool component candidate. Hereinafter, the PDEV specified in this(S12-1) will be called the “candidate PDEV”.(S12-2): The control processing program 501 selects from a plurality ofcandidate PDEV the PDEV having a total capacity greater than theuser-requested pool capacity. As a result of this, the total capacity ofthe selected PDEV (the real allocation capacity of the DP pool) iseither the same as or larger than the user-requested pool capacity.Furthermore, the RAID level and combination may also be taken intoconsideration in PDEV selection. For example, in the case where auser-requested pool capacity is 1 TB (one terabyte), the controlprocessing program 501 may select PDEV from a plurality of PDEV so thatthe total capacity of the selected PDEV is 1.3 times the user-requestedpool capacity (1.3 TB). In a case where virtual LU having different RAIDlevels and combination are associated with the created DP pool, a PDEVmay be selected on the basis of the parity ratio of the RAID level andcombination having the highest parity ratio. Further, in a case wherethe RAID level and combination of the virtual LU associated with the DPpool has been decided, the PDEV may be selected on the basis of theparity ratio of this RAID level and combination. Furthermore, the“parity ratio” is the percentage of the total size of the created paritywith respect to the size of the I/O-targeted data for the virtual page.

(S13)

The control processing program 501 adds information related to thecreated DP pool to the pool management table 505. The followinginformation is added as information related to the created DP pool.

-   -   Pool number 210: A number determined in accordance with a        prescribed rule (for example, the number after the largest        number of the pool number 210 registered in the pool management        table 505). Or, the pool number denoted by the above-mentioned        parameter (p3) of S10.    -   PDEV number list 211: The PDEV numbers selected in S12.    -   Real allocation capacity 212: The total capacity of the PDEV        selected in S12.    -   Utilization capacity 213: Zero. This is because a real page is        not allocated from the DP pool.    -   First PDEV number 214: A number determined in accordance with a        prescribed rule from among the PDEV numbers selected in S12. For        example, the smallest number of the PDEV numbers selected in        S12.

(S14)

The control processing program 501 updates the assigned pool number 231of the PDEV selected in S12 from “null” to the number of the created DPpool (the number in S13) in the PDEV management table 509.

(S15)

The control processing program 501 partitions the storage space of therespective PDEV (the respective PDEV selected in S12) configuring thecreated DP pool with respect to each real page capacity, and manages therespected real pages obtained in accordance with this. Specifically, thecontrol processing program 501 creates a real page management table 511corresponding to the created DP pool, and stores this table 511 in thememory 123. The respective records of the created real page managementtable 511 comprise the following information.

-   -   PDEV number 240: Any PDEV number selected in S12.    -   Real page number 241: Any serial number assigned to the real        pages provided by the PDEV selected in S12.    -   LUN 242: null. This is because none of the real pages has been        allocated to any of the virtual pages.    -   Virtual page number 243: null. This is because none of the real        pages has been allocated to any of the virtual pages.    -   Access counter 244: Zero. This is because an I/O is not        generated for any real page.

FIG. 15 is a flowchart showing the flow of processing for creating avirtual LU 201.

(S20)

The control processing program 501 receives a virtual LU createinstruction from the management apparatus 105. The virtual LU createinstruction, for example, has as a parameter at the least (p15) of thefollowing (p11) through (p15). The (p15) is a parameter that must beinputted by the user, and the (p11) through the (p14) are parametersthat are inputted at the discretion of the user. A parameter from amongthe parameters (p11) through (p14) that is not inputted by the user is avalue (a default value) determined in accordance with a prescribed rule.

(p11) LUN assigned to the virtual LU to be created. The default value,for example, is the smallest number of a group of numbers capable ofbeing assigned as the LUN of the virtual LU.(p12) The number of virtual LU to be created. The default value is “1”.(p13) The DP pool number of the assigned destination of the virtual LUto be created (the pool number). The default value is the smallestnumber of the created DP pool numbers.(p14) The RAID level (simultaneous setting of a combination). Thedefault number, for example, is RAID 5 (2D+1P).(p15) The capacity of the virtual LU.

(S21)

The control processing program 501 adds to the virtual LU managementtable 507 information related to the virtual LU to be created for eachvirtual LU in the number of virtual LU denoted in the above-mentioned(p12). The following information is added as information related to thevirtual LU to be created.

-   -   LUN 220: LUN denoted by the above-mentioned parameter (p11).    -   Assigned pool number 221: Pool number denoted by the        above-mentioned parameter (p13). The number of virtual LU        denoted by the above-mentioned (p12) is associated with the DP        pool having the pool number denoted by the above-mentioned        parameter (p13).    -   RAID level 222: The RAID level denoted by the above-mentioned        parameter (p14).    -   Combination 223: The combination denoted by the above-mentioned        parameter (p14).    -   Capacity 224: The capacity denoted by the above-mentioned        parameter (p15).

(S22)

The control processing program 501 adds to the mapping management table513 information related to the respective virtual pages of the virtualLU to be created for each virtual LU in the number of virtual LU denotedin the above-mentioned (p12). For example, the information added for therespective the virtual pages is as follows.

-   -   LUN 250: LUN of the virtual LU comprising the virtual page.    -   Virtual page number 251: The number of the virtual page.    -   Real page identification information 252: null. This is because        a real page is not allocated to any of the virtual pages.    -   Access counter 253: Zero. This is because an I/O is not        generated for any virtual page.

FIG. 16 shows an overview of the flow of a write to a virtual page. Theflow of a write to a virtual page for which a real page is notallocated, and the flow of a write to a virtual page for which a realpage has been allocated will be explained below.

<Case 1: Write to a Virtual Page for which a Real Page is not Allocated>

The following processing is carried out.

(w11) The control processing program 501 receives from the host #1 awrite command specifying the virtual page V11 of the virtual LU #11,references the mapping management table 513, and specifies the fact thata real page is not allocated to the write-destination virtual page V11.(w12) The control processing program 501 references the virtual LUmanagement table 507, and specifies that the RAID level of the virtualLU #11 that comprises the virtual page V11 is RAID 5 (2D+1P).(w13) The control processing program 501 references the real pagemanagement table 511 corresponding to the DP pool #11 to which thevirtual LU #11 is assigned, and selects the three real pages havingdifferent PDEV numbers 240 (the number of real pages based on the RAIDlevel (RAID 5 (2D+1P)) specified in (w12)) from the real pages for whichthe LUN 242 and the virtual page number 243 are null. The real page R11inside PDEV #11, the real page R21 inside PDEV #12, and the real pageR31 inside PDEV #13 are selected here.(w14) The control processing program 501 allocates the real pages R11,R21, and R31 selected in (w13) to the virtual page V11 and updates themapping management table 513.(w15) The control processing program 501 writes two data elementsobtained by partitioning the write-targeted data for the virtual pageV11 to two of the three real pages R11, R21, and R31, and writes theparity based on these two data elements to the one remaining real pageof these three real pages.<Case 2: Write to a Virtual Page for which a Real Page has BeenAllocated>

The following processing is carried out.

(w21) The control processing program 501 receives from the host #2 awrite command specifying the virtual page V22 of the virtual LU #12,references the mapping management table 513, and specifies the real pagethat has been allocated to the write-destination virtual page V22. Sincethe RAID level of the virtual LU #12 that comprises the virtual page V22is RAID 6 (3D+2P), five real pages are allocated to the virtual pageV22, and these five real pages are specified in (w21). Specifically, thereal page R32 inside PDEV #13, the real page R41 inside PDEV #14, thereal page R51 inside PDEV #15, the real page R61 inside PDEV #16, andthe real page R71 inside PDEV #17 are selected.(w22) The control processing program 501 writes three data elementsobtained by partitioning the write-targeted data for the virtual pageV22 among three of the five real pages R32, R41, R51, R61, and R71specified in (w21), and writes two parities based on these three dataelements to the two remaining real pages of these five real pages.

FIG. 17 is a flowchart showing the flow of processing for a write to avirtual page.

(S30)

The control processing program 501 receives a write command comprisingwrite-destination information and write-targeted data. Thewrite-destination information includes a LUN and a LBA. Thewrite-targeted data is written to the memory 123 (the cache area).

(S31)

The control processing program 501 references the mapping managementtable 513, and determines whether or not a real page has been allocatedto the virtual page (write-destination virtual page) specified from thewrite-destination information.

<A Case where the Result of the Determination of (S31) is Affirmative (aCase where the PDEV Identification Information 252 For theWrite-Destination Virtual Page is not Null)>

This corresponds to the above-mentioned Case 2. Specifically, S37 iscarried out without performing S32 through S36. That is, the controlprocessing program 501 writes the write-targeted data (for example, thetwo or more data elements and one or more parities obtained bypartitioning this data) to the real pages (for example, the real pagesR32, R41, R51, R61, and R71 of FIG. 16) that have been allocated to thewrite-destination virtual page (for example, the virtual page V22 ofFIG. 16) based on the RAID level of the virtual LU (for example, thevirtual LU #12 of FIG. 16) comprising the write-destination virtualpage. Then, the control processing program 501 performs the followingwrite status update process:

-   -   Increments by 1 the value of the access counter 253        corresponding to the write-destination virtual page;    -   Updates the virtual access frequency information (the        information inside the virtual access status table 517)        corresponding to the write-destination virtual page        (specifically, performs an update that increases the write        frequency (one minute, one hour, one day, or one week));    -   Increments by 1 the value of the access counter 244        corresponding to the respective real pages allocated to the        write-destination virtual page; and    -   Updates the real access frequency information (the information        inside the real access status table 515) corresponding to the        respective real pages allocated to the write-destination virtual        page (specifically, performs an update that increases the write        frequency (one minute, one hour, one day, or one week)).        Furthermore, the real pages allocated to the write-destination        virtual page are able to be specified from the mapping        management table 513. The virtual LU comprising the        write-destination virtual page (hereinafter, the        write-destination virtual LU) is able to be identified from the        LUN of the write-destination information. The RAID level of the        write-destination virtual LU is able to be specified from the        virtual LU management table 507.        <A Case where the Result of the Determination of (S31) is        Negative (a Case where the PDEV Identification Information 252        For the Write-Destination Virtual Page is Null)>

This corresponds to the above-mentioned Case 1. For this reason, S32through S36 are carried out, and thereafter, S37 is performed.

(S32)

The control processing program 501 specifies the RAID level 222 andcombination 223 (for example, RAID 5 (2D+1P) corresponding to thewrite-destination virtual LU (for example, the virtual LU #11 of FIG.16). The control processing program 501 determines the number K ofselected PDEV based on the specified RAID level 222 and combination 223.For example, in a case where the combination denoting the RAID level is“nD+mP (or D)” (where n is a natural number and m is an integer of 0 ormore), the number K of selected PDEV is (n+m). Specifically, forexample, in a case where the combination is “2D+1P”, K=2+1=3.

(S33)

The control processing program 501 specifies the assigned pool number221 corresponding to the write-destination virtual LU and the first PDEVnumber 214 corresponding to the pool number 210 of the same value.

(S34)

The control processing program 501 selects the same number of PDEVnumbers as the number K selected in S32 from the assigned pool number221 corresponding to the write-destination virtual LU and the PDEVnumber list 211 corresponding to the pool number 210 of the same value.At this time, the control processing program 501, first selects the samePDEV number (for example, #11) as the first PDEV number 214 selected inS33, makes this PDEV number the first PDEV number, and selects the PDEVnumbers in order from the smallest PDEV number until the number ofselected PDEV numbers reaches K. In a case where the selected PDEVnumber reaches the largest number of the PDEV number list 211corresponding to the DP pool (for example, DP pool #11 of FIG. 16) towhich the write-destination virtual LU is assigned prior to the numberof selected PDEV numbers having reached the K, the smallest number ofthis PDEV number list 211 is selected.

Furthermore, in this (S34), the PDEV numbers (PDEV) may be selected asfollows instead of selecting the PDEV number (PDEV) in accordance withthe PDEV number order.

-   -   Preferentially select the PDEV with the most free capacity 235.        In accordance with this, the PDEV in the DP pool can be expected        to be used equably.    -   Select PDEV for which either one or both of the IF type 232 and        the PDEV performance 233 are the same. In accordance with this,        I/O performance with respect to the real pages allocated to one        virtual page can be expected to be the same.

(S35)

The control processing program 501 selects one free real page from therespective PDEV specified from the PDEV numbers selected in S34.Specifically, for example, the control processing program 501 selectsany real page number 241 that corresponds to a LUN 242 and virtual pagenumber 243 that are null for each selected PDEV number based on the realpage management table 511 corresponding to the DP pool to which thewrite-destination virtual LU belongs. According to the example of FIG.16, the real page number R11 is selected for the PDEV number #11, thereal page number R21 is selected for the PDEV number #12, and the realpage number R31 is selected for the PDEV number #13.

(S36)

The control processing program 501 updates the real page identificationinformation 252 corresponding to the write-destination virtual page fromnull to a combination of the real page numbers 241 in S35 and the PDEVnumbers 240 corresponding to these numbers 241 (for example, #11-R11,#12-R21, #13-R31). That is, the control processing program 501 allocatesthe two or more free real pages selected in S35 to the write-destinationvirtual page (for example, the virtual page V11 in FIG. 16).

(S37)

The control processing program 501, based on the RAID level andcombination of the write-destination virtual LU, writes thewrite-targeted data to the two or more real pages allocated in S36.According to the example of FIG. 16, the two data elements and oneparity are written to the three real pages R11, R21, and R31. Then, thecontrol processing program 501 performs the write status update processas described hereinabove. In accordance with this, the value of theaccess counter 253 corresponding to the write-destination virtual page,the virtual access frequency information corresponding to thewrite-destination virtual page, the value of the access counter 244corresponding to the respective real pages allocated to thewrite-destination virtual page, and the real access frequencyinformation corresponding to the respective real pages allocated to thewrite-destination virtual page are updated.

FIG. 18 is a flowchart showing the flow of processing of a read withrespect to a virtual page.

(S40)

The control processing program 501 receives a read command comprisingread-source information. The read-source information, for example,comprises a LUN and a LBA.

(S41)

The control processing program 501 references the mapping managementtable 513, and determines whether or not a real page is allocated to thevirtual page specified from the read-source information (the read-sourcevirtual page). In a case where the result of the determination in S41 isnegative, S42 is carried out. In a case where the result of thedetermination in S41 is affirmative, S43 is carried out.

(S42)

The control processing program 501 sends an error to the host that isthe source of the read command.

(S43)

The control processing program 501 reads two or more data elements (andone or more parities) from the two or more real pages allocated to theread-source virtual page, and sends to the read command-source host thedata configured by the two or more data elements that have been read(that is, the read-targeted data) (the read-targeted data may be datathat includes a data element that was recovered using parity). Then, thecontrol processing program 501 performs the following read status updateprocess:

-   -   Increments by 1 the value of the access counter 253        corresponding to the read-source virtual page;    -   Updates the virtual access frequency information (the        information inside the virtual access status table 517)        corresponding to the read-source virtual page (specifically,        performs an update that increases the read frequency (one        minute, one hour, one day, or one week));    -   Increments by 1 the value of the access counter 244        corresponding to the respective real pages allocated to the        read-source virtual page; and    -   Updates the real access frequency information (the information        inside the real access status table 515) corresponding to the        respective real pages allocated to the read-source virtual page        (specifically, performs an update that increases the read        frequency (one minute, one hour, one day, or one week)).

As explained hereinabove, in this embodiment, a real page thatconfigures a DP pool 203 is not a substantial storage area provided froma RAID group, but rather is a storage area that is provided from onePDEV. Then, it is possible to associate a plurality of virtual LU withdifferent RAID level definitions in a single DP pool 203. For thisreason, as shown in the example of FIG. 19, in one DP pool #40, datarespectively stored at different RAID levels, such as a set of data(data elements a, b and c) and parity P stored, for example, at RAID 5(3D+1P) is mixed together with a set of data (data elements e, f, g, hand i) and parities P1 and P2 stored at RAID 6 (5D+2P). In accordancewith this, in a case where a RAID level or a combination defined for acertain virtual LU is changed to a different RAID level or acombination, the reconstruction of the data may be carried out only forthe real page(s) allocated to this virtual LU. For example, in a casewhere the RAID level (or combination) of the virtual LU changes from afirst RAID level (or combination) to a second RAID level (orcombination), the control processing program 501 carries out, for eachvirtual page, the following processing—(k1) to (k3)—for each virtual LUto which a real page is allocated:

(k1) Reads v data units from v real pages allocated to the virtual pageU, and constructs the data based on the v data units (a “data unit” asused in the explanation of this embodiment is a generic term for eithera data element or a parity that is stored in the real page);(k2) respectively selects from w PDEV of the DP pool to which thevirtual LU is associated w real pages on the basis of the second RAIDlevel and combination, and allocates the selected w real pages to thevirtual page instead of the v real pages of (k1); and(k3) writes the w data units based on the data constructed in (k1) tothe allocated w real pages. Furthermore, in (k2), the w real pages maybe selected so that the utilization rates of the respective PDEV in theDP pool associated with the virtual LU for which the RAID level (orcombination) changed become uniform. In accordance with this, theutilization rates of the respective PDEV that belong to the DP pool withwhich this virtual LU is associated can be expected to be made uniformas a result of changing the RAID level (or combination) that is definedfor the virtual LU. In this case, the w real pages may be selected sothat not the utilization rates of the respective PDEV but the freecapacity of the respective PDEV become uniform.

Further, in the above explanation, the source PDEV for the real page(s)allocated to the virtual page is selected in PDEV number order, but, forexample, as shown in FIG. 20, the PDEV may be selected at random.According to the example of FIG. 20, the data elements a, b, and c andthe parity P in conformance with the RAID 5 (3D+1P), and the dataelements e, f, g, h, and i and the parities P1 and P2 in conformancewith the RAID 6 (5D+2P) are stored in the real pages of the respectivePDEV that were selected without relying on the PDEV number. Furthermore,instead of relying on the PDEV number, the PDEV utilization rate (orfree capacity) may be taken into consideration. This is to make theutilization rates (or free capacity) of the plurality of PDEV in the DPpool equable.

Further, the control processing program 501 may allocate a plurality ofreal pages to one virtual page from a plurality of PDEV that are coupledto the same backend coupling device 113. In accordance with this, theaccess performance to these plurality of real pages can be expected tobe made uniform. Furthermore, in a case where the utilization rates (orfree capacity) of the plurality of PDEV that are coupled to the samebackend coupling device 113 are not equable, a plurality of real pagesmay be allocated to the virtual page from a plurality of PDEV coupled toa plurality of backend coupling devices 113.

Now then, in this embodiment, a rearrange process, by which a data unitis migrated from a real page inside a certain HDD to a real page insidea different PDEV, is performed.

For example, this rearrange process is carried out when a PDEV is addedor removed. Specifically, for example, in a case where the totalcapacity of the free real pages in the DP pool 203 is insufficient (forexample, a case in which the ratio of the utilization capacity 213 tothe real allocation capacity 212 of the DP pool 203 is not less than apredetermined threshold), the addition of a PDEV is carried out toincrease the free real pages. The addition of a PDEV to the DP pool 203may be carried out in PDEV units. Furthermore, it is possible for thecontrol processing program 501 to detect the fact that the totalcapacity of the free real pages is insufficient by referencing the poolmanagement table 505. In a case where the control processing program 501detects that the total capacity of the free real pages is insufficient,the control processing program 501 sends the management apparatus 105 awarning and the number of the DP pool in which the total capacity of thefree real pages is insufficient. The management apparatus 105 displaysthe warning and DP pool number. The user sees this warning and DP poolnumber, adds a PDEV, and makes the added PDEV a member of the DP poolidentified from the DP pool number displayed together with the warning.Furthermore, the addition of a PDEV is not limited to when the totalcapacity of the free real pages is insufficient, and may also be carriedout at other times (for example, in accordance with a prescribed plan).

FIG. 21 is a flowchart of the processing carried out when adding a PDEV.

(S1801)

In a case where a PDEV has been added (for example, a case in which thecontrol processing program 501 has detected that a PDEV has been added),the control processing program 501 adds information (a record) relatedto the added PDEV (hereinafter, the added PDEV) to the PDEV managementtable 509. Of the information related to the PDFV, the IF type 232, thePDEV performance 233, the capacity 234, and the free capacity 235portions of the information related to the PDEV may be inputted by theuser via the management apparatus 105, or may be acquired from the addedPDEV. The PDEV number 230 may be inputted by the user via the managementapparatus 105, or may be determined by the control processing program501. The control processing program 501 adds the assigned pool number231, that is, the number of the DP pool in which the added PDEV hasbecome a member to the information related to the added PDEV in the PDEVmanagement table 509. This assigned pool number 231 may be a numberinputted by the user via the management apparatus 105, or may be anumber that was determined by the control processing program 501 (forexample, the number of the DP pool in which the shortage of free realpage total capacity was detected).

(S1802)

The control processing program 501 adds the number of the added PDEV tothe PDEV number list 211 (hereinafter, referred to as the “target list211” in the explanation of FIG. 21) corresponding to the same poolnumber 210 (refer to FIG. 7) as the assigned pool number 231 added inS1801, and, in addition, adds the capacity of the added PDEV to the realallocation capacity 212 corresponding to this pool number 210. Further,the control processing program 501 adds the information (records)corresponding to the respective real pages of the added PDEV to the realpage management table 511 that corresponds to the assigned pool number231 added in S1801.

(S1803)

From among the capacity 234 and the free capacity 235 managed in thePDEV management table 509, the control processing program 501 referencesthe capacity 234 and the free capacity 235 corresponding to the PDEVthat belongs to the target list 211, and computes the utilization rate(the ratio of the free capacity 235 to the capacity 234) of therespective PDEV in the target list 211. Then, the control processingprogram 501 specifies the PDEV for which the utilization rate exceeds apredetermined threshold.

(S1804)

The control processing program 501 executes the rearrange process untilthe utilization rate of the PDEV specified in S1803 becomes less thanthe predetermined threshold. The “rearrange process” is the process formigrating a data unit from a real page inside a certain PDEV to a realpage inside a different PDEV in a single DP pool.

(S1805)

The control processing program 501 executes the rearrange process untilthe utilization rates of all the PDEV in the DP pool comprising theadded PDEV become equable. In this process executed in a case where aPDEV is added, the PDEV which is the target of the rearrange process maybe decided based on not the utilization rates of the PDEV but freecapacity of the PDEV.

FIG. 22 shows a flowchart of the rearrange process. Furthermore, in thefollowing explanation, data created based on a plurality of data units(data for which an I/O is performed with respect to a virtual area) willbe called “I/O data”.

(S1901)

The control processing program 501 selects the data unitmigration-source real page and the data unit migration-destination realpage. In a case where this rearrange process is the rearrange process inS1804 of FIG. 21, the migration-source real page is selected from thePDEV specified in S1803 of FIG. 21 (the PDEV for which the utilizationrate exceeds the predetermined threshold), and the migration-destinationreal page is selected from the PDEV for which the utilization rate doesnot exceed the predetermined threshold (for example, the PDEV with thelowest utilization rate) even when the data unit is stored in at leastone free real page. Alternatively, in a case where this rearrangeprocess is the rearrange process in S1805 of FIG. 21, themigration-source real page is selected from the PDEV with the highestutilization rate, and the migration-destination real page is selectedfrom the PDEV with the lowest utilization rate.

(S1902)

The control processing program 501 determines whether or not the RAID issupported. “Whether or not the RAID is supported” refers to whether ornot it is possible to create I/O data on the basis of RAID technology.Specifically, it refers to whether or not a different data unit, whichwill become the basis for I/O data created based on a data unit storedin the migration-source real page, exists in a different real pageinside the PDEV comprising the migration-destination real page. Morespecifically, for example, the control processing program 501 determineswhether or not the number of the PDEV comprising themigration-destination real page is included in the real pageidentification information 252 corresponding to the virtual page that isthe allocation destination of the migration-source real page, anddetermines whether the RAID is supported. In a case where the result ofthis determination is affirmative, S1903 is carried out, and in a casewhere the result of this determination is negative, S1904 is carriedout.

(S1903)

The control processing program 501 migrates the data unit from themigration-source real page to the migration-destination real page. Then,the control processing program 501 updates the identificationinformation of the migration-source real page (the combination of thePDEV number and the real page number), which is included in the realpage identification information 252 corresponding to the virtual pagethat was the allocation destination of the migration-source real page,to the identification information of the migration-destination realpage. That is, the control processing program 501 allocates themigration-destination real page to the virtual page in place of themigration-source real page.

(S1904)

The control processing program 501 selects the migration-destinationreal page from another PDEV. For example, the other PDEV may be a PDEVof which the usage ratio is in a level next lower than the PDEV havingthe migration-destination selected in S1901, regardless of whether therearrange process in FIG. 21 is in S1804 or S1805. Further, in thisrearrange process, the PDEV may be selected based on not the utilizationrates of the PDEV but free capacity of the PDEV.

The preceding is an explanation of the processing carried out when aPDEV is added. According to S1805 of FIG. 21, for example, a data unitis migrated as shown in FIG. 23. For example, it is supposed that in thePDEV #01 through #09 that belong to a single DP pool, PDEV #01 and #05store four data units, and the remaining PDEV #02 through #04 and #06through #09 store three data units. In a case where the added PDEV #10is assigned to the DP pool here, one data unit each is migrated from thePDEV #01 and #05 to the added PDEV #10. In accordance with this, theutilization rate of the PDEV #01 through #10 becomes as uniform aspossible.

The process related to adding a PDEV is not limited to the explanationgiven above, and variations such as those described below areconceivable. In this process, a PDEV may be selected based on not theutilization rates of the PDEV but free capacity of the PDEV.

(First Variation)

S1803 through S1805 are not carried out. In this variation, the controlprocessing program 501 always selects the real page, which is to beallocated to a write-destination virtual page, from the added PDEV inthe processing of each write command. For example, in a case where PDEV#51 through #53 are in DP pool #20 and PDEV #54 is added to this DP pool#20, the control processing program 501, in the processing of a writecommand, always does so such that the two or more real pages to beallocated to the write-destination virtual page comprise real pagesinside the PDEV #54. Furthermore, at all times the selection of a realpage from the added PDEV in the processing of a write command is carriedout until the utilization rate of the added PDEV is about the same asthe utilization rates of the other PDEV inside the DP pool that includesthe added PDEV (for example, either the same as the average of theutilization rates of the PDEV in the DP pool, or a difference with thisaverage of less than a predetermined value). Thereafter, the pluralityof PDEV in the DP pool are selected equably.

(Second Variation)

The migration-destination real page is determined based on the accessfrequency of the migration-source real page (the access frequencyspecified from the real access status table 515). For example, in a casewhere the I/O performance of the PDEV comprising the migration-sourcereal page is high compared to the access frequency of themigration-source real page, the migration-destination real page may beselected from the PDEV (for example, a HDD) with a lower I/O performancethan the PDEV (for example, a SSD) that comprises the migration-sourcereal page. By contrast, for example, in a case where the I/O performanceof the PDEV comprising the migration-source real page is low compared tothe access frequency of the migration-source real page, themigration-destination real page may be selected from the PDEV (forexample, a SSD) with a higher I/O performance than the PDEV (forexample, a HDD) that comprises the migration-source real page.

FIG. 24 is a flowchart of the processing carried out when a PDEV isremoved.

(S2701)

The control processing program 501 receives the input of the number ofthe PDEV targeted for removal from the user via the management apparatus105.

(S2702)

The control processing program 501 references the real page managementtable 511, and specifies one or more real pages, which are respectivelyallocated to one or more virtual pages from among the plurality of realpages of the PDEV targeted for removal, from the real page for which LUN242 and the virtual page No. 243 are not “null”.

(S2703)

The control processing program 501 supports the RAID, and, in addition,migrates data units so that the utilization rates are equable.Specifically, for example, the control processing program 501 selectsone real page (the migration-destination real page) from the PDEV withthe lowest utilization rate of the PDEV other than the PDEV targeted forremoval for each real page (migration-source real page) that wasspecified in S2702, and migrates a data unit from the migration-sourcereal page to the selected migration-destination real page. It ispreferable that the I/O performance of the PDEV comprising themigration-source real page be the same as the I/O performance of thePDEV comprising the migration-destination real page.

(S2704)

The control processing program 501 allocates to the virtual page, towhich the migration-source real page is allocated, themigration-destination real page in place of the migration-source realpage for each real page (migration-source real page) specified in S2702.Specifically, for example, the control processing program 501 updatesthe identification information of the migration-source real page to themigration-destination real page identification information in the realpage identification information 252 corresponding to the virtual page towhich the migration-source real page is allocated for each real page(migration-source real page) specified in S2702.

(S2705)

The control processing program 501 deletes the information related tothe PDEV targeted for removal from the table stored in the memory 123.Specifically, for example, the control processing program 501 performsthe following processing:

-   -   Deletes the record corresponding to the PDEV targeted for        removal from the PDEV management table 509;    -   Deletes the number of the PDEV targeted for removal from the        PDEV number list 211 corresponding to the DP pool of the        remove-targeted PDEV in the pool management table 505, and, in        addition, subtracts the capacity of the remove-targeted PDEV        from the real allocation capacity 212; and in a case where the        remove-targeted PDEV number is the first PDEV number 214, the        value for the first PDEV number 214 is updated to a value for        the PDEV number in the level next larger than the        remove-targeted PDEV number.    -   deletes the information (records) corresponding to the        respective real pages of the remove-targeted PDEV from the real        page management table 511 that corresponds to the DP pool of the        remove-targeted PDEV. In this process executed in a case where a        PDEV is removed, the PDEV may be selected based on not the        utilization rates of the PDEV but free capacity of the PDEV.

Now then, there are times when the access frequency of the specifiedreal page becomes high and the operating rate (load) of the PDEVcomprising this real page increases in accordance with the number andtypes of I/O commands and the I/O destination information received perunit of time by the storage system 103 while the storage system 103 isin operation.

Accordingly, in this embodiment, a rearrange process is carried outbased on the real access status table 515 and/or the virtual accessstatus table 517, which are updated in accordance with the processing ofan I/O command. For example, the following three examples (EX1) through(EX3) are conceivable with respect to the rearrange process.

(EX1) The control processing program 501 references the real accessstatus table 515 and/or the virtual access status table 517, anddetermines whether or not there is a PDEV (hereinafter, high-load PDEV)for which the access frequency or operating rate exceeds a firstthreshold. The PDEV access frequency, for example, is the average accessfrequency of the real pages of the PDEV. In a case where a high-loadPDEV is specified, the control processing program 501 migrates a dataunit inside the high-load PDEV to a free real page inside the PDEV withthe lowest operating rate. The real page storing the migration-targeteddata unit is either the real page that is allocated to the virtual pagewith the highest access frequency or the real page with the highestaccess frequency. According to the example of FIG. 25, the data unit “J”inside the high-load PDEV #05 is migrated to the PDEV #10 with thelowest operating rate. As a result of this, the operating rate of thehigh-load PDEV #05 is lowered from 40% to 20%, while the operating rateof the PDEV #10 is raised from 10% to 20%. It is preferable that themigration of the data unit be carried out such that the operating ratesof a plurality of PDEV become uniform.(EX2) The control processing program 501 references the real accessstatus table 515 and/or the virtual access status table 517, anddetermines whether or not there is a PDEV (hereinafter, low-load PDEV)for which the access frequency or operating rate falls below a secondthreshold (the second threshold<the first threshold). In a case where alow-load PDEV is specified, the control processing program 501 migratesa data unit inside the PDEV with the highest operating rate to a freereal page inside the low-load PDEV. The real page storing themigration-targeted data unit is either the real page that is allocatedto the virtual page with the highest access frequency or the real pagewith the highest access frequency.(EX3) The control processing program 501 references the real accessstatus table 515 and/or the virtual access status table 517, andcomputes the read ratio and the write ratio for either each real page oreach virtual page. The read ratio and the write ratio are the ratios ofreads and writes to overall accesses. Specifically, for example, theread ratio and the write ratio may be computed on the basis of the readfrequency and the write frequency, and the total of the read frequencyand the write frequency. The control processing program 501, in a casewhere the condition of either (X1) or (X2) below is met, migrates a dataunit inside the real page comprising either the computed read ratio orthe computed write ratio to a real page comprising an attribute (forexample, I/O performance) that is suitable for either the computed readratio or the computed write ratio.(X1) Either the computed read ratio or the computed write ratio does notconform to the attribute (for example, the I/O performance) of the PDEVproviding the real page that comprises either this read ratio or thiswrite ratio.(X2) Either the computed read ratio or the computed write ratio does notconform to the attribute (for example, the I/O performance) of the PDEVproviding the real page that is allocated to the virtual page thatcomprises either this read ratio or this write ratio.

There are times when the assigned destination of the virtual LU isswitched from a first DP pool to a second DP pool. In accordance withthis, the following (f1) and (f2) processes are carried out for eachvirtual page to which a real page inside the first DP pool is allocated:

(f1) Data units are respectively migrated from the plurality of realpages inside the first DP pool, which are allocated to the virtual page,to the plurality of real pages inside the second DP pool; and(f2) the plurality of migration-destination real pages are allocated tothe virtual page in place of the plurality of migration-source realpages. Furthermore, the migration-destination real pages in (f1) areselected from the PDEV that comprises the attribute (for example, I/Operformance) that conforms to the access frequency of the virtual page(the access frequency specified from the virtual access status table517). For example, in a case where the access frequency of the virtualpage is greater than a predetermined threshold, themigration-destination real page may be selected from the PDEV with highI/O performance, and alternatively, in a case where the access frequencyof the virtual page is lower than the predetermined threshold, themigration-destination real page may be selected from the PDEV with lowI/O performance.

As described hereinabove, in this embodiment, the real page of the DPpool is provided from the PDEV rather than from the RAID group. For thisreason, in a case where a failure occurs in the PDEV, a data unitrestore need only be carried out for the real page allocated to thevirtual page from among the plurality of real pages of the PDEV in whichthe failure occurred (hereinafter, the failed PDEV). Here, theexpression “restore” means the restoration of data stored in the realpage in the failed PDEV from the data elements or parity written in thereal pages other than the real page in the failed PDEV from among realpages assigned with the same real page. Specifically, for example, in acase where only data element “b” is stored in the failed PDEV as shownin FIG. 26, the control processing program 501 restores the data element“b” on the basis of the other data elements “a” and “c” and the parity“P”, and writes the restored data element “b” to a PDEV in which afailure has not occurred. The PDEV that becomes the write destination isselected such that the utilization rates (or free capacity) in the DPpool are equable.

As processes related to a data unit restore inside a failed PDEV, theremay be a priority-restore type restore process and a priority-replacetype restore process.

The priority-restore type restore process, for example, is as follows.That is, the control processing program 501 restores data units insideone or more real pages allocated to one or more virtual pages within thefailed PDEV, and distributes the restored one or more data units to oneor more other PDEV. After the failed PDEV has been replaced with anotherPDEV, the control processing program 501 migrates the data units fromthe plurality of other PDEV to the post-replace PDEV by carrying out arearrange process such that the utilization rates of the plurality ofPDEV are uniform.

The priority-replace type restore process, for example, is as follows.That is, the failed PDEV is replaced with another PDEV. The controlprocessing program 501 restores data units inside one or more real pagesallocated to one or more virtual pages within the failed PDEV, andwrites the restored one or more data units to the post-replace PDEV.

These respective restore processes will be explained in detail below.

FIG. 27 is a flowchart of the priority-restore type restore process.

(S2501)

The control processing program 501 references the mapping managementtable 513 and specifies an allocated real page within the failed PDEV.The control processing program 501 carries out (S2502) through (S2504)below for each allocated real page that is specified.

(S2502)

The control processing program 501 reads a data unit from one or moreother real pages allocated to the allocation-destination virtual page ofthe allocated real pages.

(S2503)

The control processing program 501 restores the data unit to be storedin the real page inside the failed PDEV based on the one or more dataunits read in S2502.

(S2504)

The control processing program 501 supports the RAID, and, in addition,selects a free real page as the write-destination for the data unitrestored in S2503 from another PDEV inside the DP pool such that theutilization rates of the PDEV inside this DP pool of the failed PDEV areuniform, and writes the restored data unit to the selected real page.The control processing program 501 allocates this write-destination realpage to the virtual page in place of the real page inside the failedPDEV.

(S2505)

After carrying out S2502 through S2504 for all of the allocated realpages in the failed PDEV, the failed HDD is replaced with another HDD.

(S2506)

The control processing program 501 migrates the data unit from theplurality of other PDEV inside the DP pool to the post-replace PDEV suchthat the utilization rates of the PDEV inside the DP pool are uniform.The control processing program 501 allocates the migration-destinationreal page inside the post-replace PDEV to the virtual page in place ofthe data unit migration-source real page. In the priority-replace typerestore process, a migration-destination PDEV may be selected so thatnot the utilization rates of the PDEV but free capacity of the PDEVbecome uniform.

The following is conceivable as a variation of this restore process. Forexample, in a case where the control processing program 501 receives aread command from the host 101, and, in addition, detects the fact thata real page inside the failed PDEV is included in the plurality of realpages that are allocated to this read-source virtual page, the controlprogram 510 may carry out S2503 and S2504 on the basis of the one ormore data units that were read in response to the read command. AfterS2503 and S2504 have been carried out for all the allocated real pagesin the failed PDEV, S2505 and S2506 may be carried out.

FIG. 28 is a flowchart of the priority-replace type restore process.

(S2601)

The failed PDEV is replaced with another PDEV.

(S2602)

The control processing program 501 references the real page managementtable 511, and specifies an allocated real page within the failed PDEV.The control processing program 501 carries out (S2603) through (S2605)below for each allocated real page that is specified.

(S2603)

The control processing program 501 reads a data unit from one or moreother real pages allocated to the virtual page that is the allocationdestination of the allocated real page.

(S2604)

The control processing program 501 restores the data unit to be storedin the real page inside of the failed PDEV on the basis of the one ormore data units read in S2603.

(S2605)

The control processing program 501 writes the restored data unit to thepost-replace PDEV.

The decision to perform a priority-restore or a priority-replace may bemade in accordance with the access status with respect to the storagesystem 103, or may be preset by the user.

The preceding has been an explanation of one embodiment of the presentinvention, but, needless to say, the present invention is not limited tothis embodiment, and a variety of changes are possible without departingfrom the scope thereof. For example, each element may be identified byanother type of identification information either instead of or inaddition to a number and a name. Further, for example, the creation of aparity may be carried out by a hardware circuit instead of the controlprocessing program 501. Further, for example, at the least one of theaccess counter 244 for each real page and the access counter 253 foreach virtual page may be at the least one of a write counter and a readcounter instead of the access counter. Further, for example, the realaccess status table 515 may be eliminated. In this case, it is possibleto regard the access frequency of the virtual page as the accessfrequency of the real page corresponding to the virtual page.

REFERENCE SIGNS LIST

-   103 Storage system

1. A storage system coupled to a host apparatus, the storage systemcomprising: a controller; and a plurality of storage devices including afirst type of storage devices and a second type of storage devices, anattribute of the first type of storage devices being different from anattribute of the second type of storage devices, the controllerconfigured to: manage at least a pool including a plurality of realpages, each of which is included in one of the plurality of storagedevices; provide a virtual volume to the host apparatus, the virtualvolume including a plurality of virtual pages, to each of which aportion of the pool is allocated in accordance with a write command;distribute data written in a first virtual page to a first group of realpages, the first group of real pages being included in a portion of thepool allocated to the first virtual page, the first group of real pagesmaking up a redundant array and being selected from different storagedevices of the first type of storage devices; and migrate data stored inthe first group of real pages to the second type of storage devices sothat each real page of the first group belongs to the second type ofstorage devices according to an access status of the first virtual page.2. The storage system according to claim 1, wherein the controller isconfigured to distribute data written in a second virtual page to asecond group of real pages, the second group of real pages beingincluded in a portion of the pool allocated to the second virtual page,the second group of real pages making up a redundant array and beingselected from different storage devices of the first type of storagedevices; wherein a performance attribute of the first type of storagedevices is higher than a performance attribute of the second type ofstorage devices, and the access frequency of the first virtual page ishigher than an access frequency of the second virtual page, and whereinthe controller is configured to not migrate data stored in the secondgroup of real pages when the data stored in the first group of realpages is migrated.
 3. The storage system according to claim 1, whereinthe controller is configured to: distribute data written in a secondvirtual page to a second group of real pages, the second group of realpages being included in portion of the pool allocated to the secondvirtual page, the second group of real pages making up a redundant arrayand being selected from different storage devices of the first type ofstorage devices; migrate data stored in a first real page, which is areal page of the second group of real pages and belongs to a firststorage device of the first type of storage devices, to another storagedevice of the first type of storage devices when the load of the firststorage device is high; and not migrate data stored in another real pageof the second group of real pages and data stored in another real pagebelonging to the first storage device.
 4. The storage system accordingto claim 1, wherein the first storage device stores data of the secondvirtual page and data of a third virtual page, and wherein a secondstorage device stores data of the second virtual page and does not storedata of the third virtual page.
 5. The storage system according to claim1, wherein a number of real pages of the first group is predeterminedbased on a number of data and parity data designated to the virtualvolume.
 6. The storage system according to claim 3, wherein when theload of the first storage device is high, the controller is configuredto migrate the data stored in the first real page.
 7. The storage systemaccording to claim 3, wherein the controller is configured to migratethe data stored in the first real page to another storage device whichdoes not include the real pages of the first group.
 8. A method in astorage system coupled to a host apparatus and having a controller and aplurality of storage devices including a first type of storage devicesand a second type of storage devices, an attribute of the first type ofstorage devices being different from an attribute of the second type ofstorage devices, the method comprising the steps of: managing, by thecontroller, at least a pool including a plurality of real pages, each ofwhich is included in one of the plurality of storage devices; providing,by the controller, a virtual volume to the host apparatus, the virtualvolume including a plurality of virtual pages, to each of which aportion of the pool is allocated in accordance with a write command;distributing, by the controller, data written in a first virtual page toa first group of real pages, the first group of real pages beingincluded in a portion of the pool allocated to the first virtual page,the first group of real pages making up a redundant array and beingselected from different storage devices of the first type of storagedevices; and migrating, by the controller, data stored in the firstgroup of real pages to the second type of storage devices so that eachreal page of the first group belongs to the second type of storagedevices according to an access status of the first virtual page.
 9. Themethod according to claim 8, further comprising the steps of:distributing, by the controller, data written in a second virtual pageto a second group of real pages, the second group of real pages beingincluded in a portion of the pool allocated to the second virtual page,the second group of real pages making up a redundant array and beingselected from different storage devices of the first type of storagedevices, wherein a performance attribute of the first type of storagedevices is higher than a performance attribute of the second type ofstorage devices, and the access frequency of the first virtual page ishigher than an access frequency of the second virtual page, and notmigrating, by the controller, data stored in the second group of realpages when the data stored in the first group of real pages is migrated.10. The method according to claim 8, further comprising the steps of:distributing, by the controller, data written in a second virtual pageto a second group of real pages, the second group of real pages beingincluded in portion of the pool allocated to the second virtual page,the second group of real pages making up a redundant array and beingselected from different storage devices of the first type of storagedevices; migrating, by the controller, data stored in a first real page,which is a real page of the second group of real pages and belongs to afirst storage device of the first type of storage devices, to anotherstorage device of the first type of storage devices when the load of thefirst storage device is high; and not migrating, by the controller, datastored in another real page of the second group of real pages and datastored in another real page belonging to the first storage device. 11.The method according to claim 8, further comprising the steps of:storing, by the first storage device, data of the second virtual pageand data of a third virtual page, and storing, by a second storagedevice, data of the second virtual page and not storing data of thethird virtual page.
 12. The method according to claim 8, wherein anumber of real pages of the first group is predetermined based on anumber of data and parity data designated to the virtual volume.
 13. Themethod according to claim 10, further comprising the step of: when theload of the first storage device is high, migrating, by the controller,the data stored in the first real page.
 14. The method according toclaim 10, further comprising the step of: migrating, by the controller,the data stored in the first real page to another storage device whichdoes not include the real pages of the first group.